Physicist Hertha Ayrton made significant improvements to the efficiency and design of direct current electric arc (a source of bright illumination), and wrote a widely respected textbook on the topic. She discovered the relationship between pressure in the arc and current length, and later studied the fluid dynamics of waves. In 1884 she invented the line divider, used by artists, architects, engineers, and surveyors to precisely divide a line into equal sections, and during World War I she invented the Ayrton flapper fan, used on the battlefront to dissipate poison gas attacks, and later adapted to improve ventilation for mine-workers.

Born Phoebe Sarah Marks and raised Jewish, in her teens she declared herself agnostic and adopted the flamboyant name Hertha, after a Teutonic earth goddess featured in an anti-religious poem by Algernon Charles Swinburne. She studied mathematics at Cambridge and completed all requirements for a degree, but women at the time were allowed only to attend, not graduate at Cambridge, so she instead received her degree by special test at the University of London. In 1899 she became the first female member of the Institution of Electrical Engineers (now the Institution of Engineering and Technology); at her death in 1923 she was still the IEE's only female member. She was the first woman to address the Royal Society, the first woman to win that group's Hughes Medal, and her husband -- the physicist William Edward Ayrton -- was a member, but when she was nominated for membership in 1902, she was rejected tersely: "We are of the opinion that married women are not eligible as Fellows of the Royal Society."

An outspoken suffragette, Ayrton was a frequent presence at women's rights rallies. She was a long-time friend of Marie Curie, and in 1912 she took the ill Curie and Curie's daughters into her home for several months. Her daughter, Barbara Ayrton Gould, ran for the House of Commons and lost seven elections, before winning a seat on her eighth run in 1945. Her stepdaughter, author and feminist Edith Ayrton Zangwill, was married to writer and Zionist Israel Zangwill. At her death Ayrton left the bulk of her estate to the IEE.